The present invention relates generally to a photomask used for fabricating high-density integrated circuits such as LSIs and VLSIs and a photomask blank used for fabricating such a photomask, and more particularly to a phase shift photomask and a blank therefor.
In phase shift lithography it is essentially required to control a phase shift angle with high precision so as to improve resolution and contrast. This phase shift angle depends on the thickness and refractive index of a phase shift pattern. Of grave concern in the fabrication of a phase shift photomask is thickness control. In most cases, dry etching that is an excellent high-precision processing technique is used for etching a phase shifter layer. The surest technique for controlling the thickness of a phase shifter pattern is an over-etching process wherein the phase shifter and underlying layers are made up of such a combination of materials that the etch rate ratio (selectivity) of those layers is increased, and dry etching is carried out for a period of time that is longer than that which would be taken to remove the phase shifter layer completely in a given length of time. This assures that uniform etching depth is achieved over the surface of the substrate with good enough precision. In addition, over-etching is required for keeping the section of the pattern in good shape and reducing a variation of the dimension of the substrate processed.
Materials, of which phase shift photomasks are formed, must be transparent to exposure light, resistant to chemicals used for fabricating and processing them, stable in the environment in which they are used, and inexpensive. From these viewpoints, it is general that synthetic quartz of high purity is used for a substrate material, while spin-on glass or an SiO.sub.2 or other film is used for a phase shifter material.
However, a problem with the use of commercially available spin-on glass (Accuglass 211S, Allied Signal Corp.), for instance, is that its dry etch selectivity with respect to a quartz substrate is at most about 1.5 to 2.0; the quartz substrate is etched too deep to have an adverse influence on a phase shift angle, even when the necessary yet minimum over-etching is carried out. Even with an SiO.sub.2 film formed by thin-film deposition techniques such as sputtering, it is impossible to prevent etching of the quartz substrate.
While a search has been made for a combination of substrate and phase shifter materials that has an increased etch selectivity, never until now has any combination that is well satisfactory in terms of the above properties been found.
To control the phase shift angle with good-enough precision, it is thus inevitably required that the substrate be provided thereon with a so-called etching stopper layer which is made up of material having an increased etch selectivity with respect to the phase shifter layer. A transparent, electrically conductive film composed predominantly of SnO.sub.2 and a transparent insulating film composed predominantly of Al.sub.2 O.sub.3, MgF.sub.2, etc., are known to play an etching stopper role.
However, a problem with the transparent conductive film composed mainly of tin oxide was that it is less than satisfactory in terms of transparency to light used for transfer. For instance, this film is almost opaque to light of 248 nm wavelength, and so cannot be used for KrF excimer laser lithography. Even for i-line exposure (365 nm), film thickness must be limited to about 15 nm because a large part of i-line (365 nm) from a mercury lamp is absorbed.
Moreover, this transparent conductive film composed primarily of tin oxide, because of being less resistant to dry etching, is completely removed by an about 20-minute over-etching, if it is about 15 nm in thickness; in other words, it fails to play an etching stopper layer role. Thus, a phase shift photomask fabricated from such a blank makes precise phase control difficult, because it is impossible to carry out sufficient over-etching when etching the phase shifter layer.
Material composed primarily of Al.sub.2 O.sub.3, MgF.sub.2, etc., and in the form of a single crystal as an example, on the other hand, is excellent in transparency to light of short wavelength, resistance to dry etching and chemical stability. However, much difficulty is involved in forming a film having satisfactory properties on a quartz substrate by means of thin-film deposition techniques such as sputtering.
This will now be explained with reference to Al.sub.2 O.sub.3 as an example. An Al.sub.2 O.sub.3 film formed by high-frequency sputtering using sintered Al.sub.2 O.sub.3 as a target material in argon is excellent in transparency to light of short wavelength and resistance to dry etching but, for lack of chemical stability, is readily dissolved in acids used at a photomask-washing step. This film is also less resistant to chemical liquids used in the patterning process. To obtain chemical stability close to that of the single crystal Al.sub.2 O.sub.3 mentioned above, the Al.sub.2 O.sub.3 film may be heated at a temperature elevated to 600.degree. C. or more. However, this heat treatment places a grave hurdle on the way to achieve high productivity, etc.
An MgF.sub.2 film obtained by sputtering, too, has unsatisfactory optical properties and chemical stability for lack of fluorine atoms, although it excels in resistance to dry etching. A film obtained by vacuum evaporation does not stand up to practical use due to its poor adhesion to an associated substrate, although it is considerably improved in terms of optical properties and chemical stability.